Antenna technology has been developed for the transmission and reception of electronic signals in a wide range of devices, including radar and communications devices. Depending on the particular device and its functional purposes, signals may be data, audio, visual, or other types of signals. Many devices that utilize antenna technology involve interaction with additional electronic components. Since these components are frequently packaged together with the antenna to form a single device, the size and weight of the antenna module has important implications for the interior design and the manufacturing process of such devices. This is especially true for airborne and space applications, where platforms are constrained by having limited surface areas for antenna mounting. Given that smaller models are generally more convenient to use, more marketable, and can be less expensive to build, small antenna modules with high gain, high power efficiency, and wide bandwidth are extremely desirable within a large number of technological fields.
It is especially desirable to provide an antenna that is electrically small. ‘Electrically small’ is herein defined to accord with conventional definitions. That is, an electrically small antenna is an antenna with a total height of less than one quarter of a wavelength at its center frequency.
Broadband antennas having low profile designs can offer certain advantages, including being smaller, lighter weight, and easier to manufacture. Low profile broadband antennas generally consist of one or more radiating apertures located within a single transverse plane and arranged in a particular configuration. Some common examples include patch antennas, bow-tie antennas, dipole antennas, slot antennas, and spiral antennas. These antennas can be backed by a cavity and a ground plane to improve directivity and impedance match. All systems can benefit from smaller, lighter antennas. Consumer demands continue to place further size and weight requirements on antenna modules.
Existing antennas do not meet the technological needs for numerous commercial and military applications such as weather radar, Earth science radar, automotive radar, wireless communications, radio frequency identification, military security, surveillance and communication, and others. The need for improved antennas has become especially demanding in airborne and space applications where large or heavy antennas can greatly impede desired functionality. This is true for antenna applications in planes, autonomous vehicles, and satellites, among others. However, due to physical restrictions on the existing technology, most antennas are limited in their minimum occupied space without producing additional undesired distortion and interference. One physical constraint is the λ/4 requirement for cavity-backed antennas. This requirement dictates that the cavity between an antenna and a ground plane must have a height of at least λ/4 to avoid image fields interference. As a result, size reduction below λ/4 can generally only be achieved at the expense of gain, bandwidth and efficiency. Many cavity-backed antennas meeting this λ/4 cavity requirement do not permit radar or communications devices to efficiently utilize interior space. Furthermore, such antennas may be relatively heavy and expensive to produce.
Some suggestion has been made that electrically small antennas made from metamaterials may be possible. Metamaterials are artificial structures that are appealing for the application of antennas because they can be designed to exhibit electromagnetic properties not commonly found in nature. The effective permittivity and permeability of these materials can be tailored to control wave propagation through the metamaterials in desired ways. In all metamaterials, including electromagnetic band gap (EBG) metamaterials, wave propagation is determined by band structure. Metallo-magnetic and metallo-dielectric EBG metamaterials can be made of periodically-spaced metallic scatterers embedded in otherwise RF-transparent magnetic and dielectric materials. The periodic structure produces forbidden frequency bands in which electromagnetic waves of certain frequencies cannot pass. Additionally, the EBG surface approximates a perfect magnetic conductor (PMC) surface at which energy is reflected in phase with the incident wave. The usable bandwidth of the EBG when operating as a PMC is considered to be the frequency range over which the phase of the reflection coefficient is bounded by ±45°.
Given their reflecting properties as a PMC, metallo-dielectric EBG materials are well-suited for realizing electrically small yet efficient antenna designs. However, at least one obstacle to using EBG metamaterials for broadband antenna applications is the relatively narrow band gaps of the EBG, which restricts antenna bandwidth to 10% or less. This is due to the resonant nature of capacitive patches. Furthermore, EBG metamaterials are typically anchored by inductive vias, which can impose additional limitations on bandwidth in some applications.